1. Field of the Invention
The present invention relates to a signal detection apparatus for detecting original digital information from a pulse code modulated (PCM) signal transmitted through a communication channel or reproduced from a recording medium.
2. Description of the Prior Art
As the method for detecting signal from a PCM signal transmitted through a communication channel or reproduced from a recording medium, for example, it is known to detect an original digital signal after partial-response equalization to cause a specific intersymbol interference preliminarily (e.g. H. Osawa et al., "Performance Analysis of Partial Response Systems for Nonreturn-to-Zero Recording," IEEE Transactions on Magnetics, Vol. MAG-22, No. 4, July 1986). When detecting original digital information from a signal after partial-response equalization, a method of using a Viterbi decoding which is one of the maximum-likelihood decoding methods is known (e.g. H. Kobayashi, "Application of Probablistic Decoding To Digital Magnetic Recording Systems," IBM Journal of Research and Development, Vol. 15, No. 1, January 1971, pp. 64-74). In these methods, it is possible in a channel having a large intersymbol interference such as a magnetic recording channel to detect signals at a low bit error rate, in comparison with the method of decoding the original digital information by equalizing so as to suppress the intersymbol interference. An example of applying this detecting method to a digital VCR is disclosed by C. Yamamitsu et al. in "An Experimental Study on Bit Rate Reduction and High Density Recording for a Home-use Digital VTR," IEEE Transactions on Consumer Electronics, Vol. 34, No. 3, August 1988, pp. 588-596.
Incidentally, to detect original digital information from a partial-response equalized signal, the amplitude at the moment of detecting the equalized signal must be kept constant. Fluctuations of signal amplitude will increase the detection error. Accordingly, to keep constant the signal amplitude, an automatic gain control (AGC) circuit is used. The AGC circuit detects the envelope level of the equalized signal, and it is designed to control the gain controlled amplifier so as to keep the detected envelope level constant.
However, the level of the envelope does not always accurately express the amplitude at the detecting moment of the signal, but varies with the content of the transmitted digital information sequence. Therefore, in such method of keeping constant the level of the envelope, the detection error may be increased.